The process of generating a binary image from a continuous-tone image is often referred to as screening and is often implemented by comparing continuous-tone pixel values with those of a dither matrix of threshold values, commonly referred to as “screen”.
Stochastic screening, otherwise known as frequency-modulated screening or dispersed-dithered screening, is based on producing a well-dispersed pattern of isolated dots at spaced pixel locations, the average surface coverage by these dots being equal to a continuous-tone image gray level value represented in a specific location.
Various methods exist for generating stochastic screens. They can be generally divided into two broad categories: data-dependent ones (such as “error diffusion” methods) and data-independent ones, based on repetitive matrix patterns.
Data-dependent screening methods, being easy to implement and computationally inexpensive, exhibit quasi-periodic patterns visible to the human eye; when used on high-resolution imaging devices, the spatial frequencies of prevailing local quasi-periodic patterns are such that the resulting effect provides an unpleasant visible appearance. Such methods are therefore used mostly on low-resolution printing devices.
Repetitive matrix-based methods are better suited for high-resolution imaging devices, such as imagesetters, platesetters, digital printing presses and high-resolution inkjet printers. Several different factors reduce image quality when using repetitive matrix-based stochastic screens.
First, high-resolution frequencies, present in the pixel distribution inside the matrix, contribute to an overall “noisy” appearance of the image, especially on relatively large areas with a uniform gray level.
Second, practically applicable repetitive matrix sizes are such that at high output resolutions the spatial frequency of the matrix repetition produces a highly visible and unpleasant pattern. This is caused by local non-uniformities, when multiplied step-and-repeat fashion in two orthogonal directions.
Third, the existence of local non-uniformities may depend on the imaged media properties, since imaged pixel growth beyond its nominal border is affected both by media characteristics and by its environment in terms of imaged and non-imaged area, due to pixel-to-pixel interaction.
Thus, there is a desire to construct a halftone screen, which minimizes undesirable imaging artifacts for a given imaging media while maintaining the favorable characteristics of a stochastic screen.